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Causes of Long QT Syndrome (LQTS)

By Dr Ananya Mandal, MD

Long QT syndrome (LQTS) is a condition that affects the electrical activity of the heart. It is a rare condition and is most commonly inherited from first degree relatives.

Several different genetic problems may cause the condition. The syndrome affects about one in 10,000 people and may be diagnosed on electrocardiogram (ECG or EKG).

The first symptom may be a fainting spell that may also lead to sudden cardiac death following severe arrhythmia (disturbances of heart rate and rhythm). The exact cause of this arrhythmia is as yet unknown.

However, causes and risk factors associated with LQTS can be broken into genetic causes or acquired causes. (1-6)

Genetic causes

The condition may be inherited at birth or within the womb and lasts for a lifetime. Faulty genes may lead to this condition.

Heart muscle cells contain tiny pores of ion channels. As the signal for a heartbeat generates in the top of the heart, it travels down the heart muscles to the bottom of the heart. This signal causes the ion channels to open or close and allows the inward and outward movement of ions like sodium, potassium and calcium.

The genes that are defective in LQTS control the production of certain types of ion channels of the heart. This may lead to too many or too few channels that do not function adequately.

This causes the duration of the ventricle to reorganize and pump blood to increase and this shows up as a longer QT interval on EKG. Some types of LQTS involve faulty or lacking potassium ion or sodium ion channels.

There are seven known types of inherited LQTS (types 1 to 7). The most common types are LQTS 1, 2, and 3.

Those with LQTS 1 or 2 have a faulty potassium ion channel in their heart leading to arrhythmias on exercise or during strong stressful situations.

In LQTS 3, the defect lies within the sodium ion channels. This leads to arrhythmias when the heart rate is slow e.g. when the person is asleep.

The genes involved in LQTS are various and new ones are being discovered and implicated in the condition. For example, it has been found that one family may have an abnormal gene called the KVLQT1 inherited on chromosome 11, while another family may have a different abnormal gene called SCN5A inherited on chromosome 3.

QT syndrome has an inheritance pattern called autosomal dominant. This means that there is a 50 percent chance of inheriting the abnormal gene and it is unrelated to sex/gender. So, if one parent has the defective gene on one of the two chromosomes the child has a 50% chance of getting the disease.

This can be explained by the fact that one of the chromosomes from the father or mother (affected parent) is inherited by the child. If this is the defective one, the child gets the condition and if it is the healthy one, the child does not. The chances of getting the disease thus are 50%.

Acquired causes of Long QT syndrome

More than 50 medicines and some conditions can cause LQTS. Medications that have been implicated in LQTS include:

  • allergy medications like Antihistamines and decongestants,

  • diuretic pills (water pills to remove excess water used in heart failure and high blood pressure),

  • some antibiotics,

  • some antidepressant and antipsychotic medications,

  • cholesterol reducing medicines

  • diabetes medicines

  • some medications used to treat arrhythmias or abnormal heart rhythms

These patients may not have symptoms of LQTS unless they take medicines that lengthen the QT interval or lower potassium levels in the blood.

In addition severe diarrhea and vomiting that causes a major loss of potassium or sodium ions from the body may also lead to LQTS. This is however a reversible condition and replenishment of the ions restores the patient.

Further eating disorders anorexia nervosa and bulimia and severe thyroid disorders also lead to loss of potassium and may give rise to LQTS.

Reviewed by , BA Hons (Cantab)

Further Reading

Last Updated: Aug 12, 2012

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